Electroplated plastics have found widespread applications. Disadvantageously, non-conductive plastics do not lend themseleves well to the electroplating process. In fact, electroplating metal coatings onto nonconductive plastics is a complex process that requires extensive pretreatment to render the surface conductive. In "Plating of Plastics", 1971, chapter 3, pages 15-21, Mueller and Baudrand describe the principles of plating on plastics. Three stages are necessary in order to electroplate plastics. First is the preparation of the surface of the plastic to receive a conductive coating. Second is the application of the conductive layer itself. The final stage is the actual electroplating process. The first stage, surface preparation entails procedures to make the plastic surface water wettable for proper metal deposition, e.g., chemical etching, conditioning, mechanical roughening and sensitising. All of these treatments are time-consuming and contribute significantly to the manufacturing costs.
As an alternative to surface pretreatment abovedescribed, conductive fillers, e.g., carbon black, metal powder of flake, have been blended with the polymer before the part or article is fabricated. Such blending adds to the complexity and cost of the manufacturing operation. Furthermore, in order to achieve sufficient electrical conductivity for plating, high filler contents are often required. In the case of additives, such as carbon black or other powders, the structural properties of the article may be adversely affected. In other cases, the fillers are quite expensive, e.g., graphite fiber.
In still yet other methods, polymeric fibers, films, sheets and the like have been treated to make them somewhat electrically conductive. Such treatments are described to be useful for polymers having pendant cyano groups, e.g., polyacrylonitrile or modacrylics, mercaptan groups, thiocarbonyl groups, quaternary ammonium groups and the like. Representative of such techniques are those of Tomibe et al., U.S. Pat. Nos. 4,336,028; 4,364,739; 4,410,593; 4,556,507; and 4,556,508. In '508, there are disclosed products which are characterized as electrically conducting materials comprising a polymer or copolymer selected from polyesters, aromatic polyamides, polyurethanes, polycarbonates, polyamides, polyamideimides, polyphenols, polyethers, polyvinylalcohols and polyethylene oxides, suitably treated, if necessary, e.g., with dicyandiamide, or by grafting with acrylonitrile, to make them receptive to a sulfide of a metal selected from copper, cobalt, tin, mercury or lead, alone, or in combination with a sulfide of a metal selected from silver, gold and elements of the platinum group. At Col. 2, lines 50-56, it is stated that the materials may be in the form of a powder or in the form of a shaped body such as a film, plate, fiber, fabric, paper, sheet, block, pellet, thread, string, rod or pipe. There is absolutely no hint or suggestion that such materials can be used as substrates for electroplating or that they do not need to be etched prior to subsequent electroplating.
It has now been unexpectedly discovered that if such shaped polymeric articles are made to contain metallic sulfide, e.g., copper sulfide, to make them electrically conductive and then electroplated, there is no need to elaborately condition them by surface pretreatments and the use of conductive fillers can be avoided. The metal coatings obtained will, however, have the important advantages of being thin, relatively uniform and firmly adherent.
It has also been discovered that polymeric articles can be rendered conductive and then electroplated with metal in a pattern. Such can then be used in special electronics applications, e.g., printed circuit boards and flexible plastic circuitry for electronic devices. These metal-coated articles are highly advantageous for one or more reasons. First, good electrical conductivities are achieved with a simple, inexpensive process and starting materials. Secondly, because only the surface of the polymeric substrate is reacted, the physical and mechanical properties of the plastic are not diminished. Thirdly, because the conductive material is added after the polymeric substrate has been formed into a shaped article, conventional processing operations, such as casting, molding, and extruding can be carried out.